This is a randomized, double-blind clinical trial involving subjects with prolonged prior experience with zdv/ddi or zdv/ddc combination therapies. Eligible subjects were participants in actg 175 who remained on their initially assigned combination therapy until the end of follow-up in actg 175 or were crossed over to one of the combination therapies (because of a 50% decline from baseline in cd4+ cell count or the development of major HIV-related disease) they must also have remained on that combination therapy for any intervening period between the end of data collection for actg 175 (Feb 28, 1995) and entry into this study. Subjects were randomized to one of three treatment arms; either to continue their zdv/ddi or zdv/ddc, or to add 3tc to their zdv/ddi or zdv/ddc or to switch to zdv/3tc. The study was partially blinded in that subjects took either ddi or ddi placebo, or ddc or ddc placebo, as well as 3tc or 3tc placebo. The primary outcome measure was change in HIV RNA copy number. The duration of study treatment was planned to be 48 weeks.